Gas sensing element and method for making same

ABSTRACT

Sensing composition precursors and sensing compositions, useful in sensing the concentration of a gas, e.g., carbon dioxide, in a medium, e.g., blood, are disclosed. In one embodiment, the sensing composition precursor comprises a dispersed aqueous liquid including a sensing component dissolved therein, a polymer precursor, and a hydrophilic dispersing agent selected from hydroxyalkyl celluloses and mixtures thereof in an amount effective to facilitate maintaining the dispersed aqueous liquid.

This application is a division of application Ser. No. 496,560, filedMar. 20, 1990, now U.S. Pat. No. 5,175,016.

BACKGROUND OF THE INVENTION

This invention is related to gas sensing elements useful in sensing theconcentration of a gas in a medium, and to methods for making suchsensing elements. In particular, the invention relates to gas sensingelements useful in sensing the concentration of a gas, for example,carbon dioxide, in an aqueous medium, for example, blood.

In many situations, it is useful to determine the concentration, e.g.partial pressure, of a gas in a medium, e.g., a fluid medium. One suchsituation is the determination of the concentration of gas, e.g., carbondioxide, oxygen and the like, in blood. Substantial recent interest hasbeen focused on performing such determinations on a real time basis sothat the current status of the patient being treated can be effectivelymonitored. Fluorescence-based sensors have been utilized to accomplishreal time blood gas sensing. For example, fluorescence-based sensors canbe used in an extracorporeal blood loop as shown in Cooper U.S. Pat. No.4,640,820 and in vivo as disclosed in Lubbers et al Reissue U.S. Pat.No. 31,879. Each of these patents is incorporated by reference in itsentirety herein.

Yafuso et al U.S. Pat. No. 4,824,789 discloses a sensing composition andsensor useful for determining the concentration of a gas in blood. Thispatent discloses a sensing composition which is useful when placed onthe optical surface of an optical fiber to form a sensor. The sensingcomposition comprises an aqueous first phase dispersed in a cross-linkedpolymeric second phase. The aqueous first phase includes a dye, forexample a pH sensitive dye. The cross-linked polymeric second phase,which is gas permeable and ion impermeable, preferably is silicone-basedincludes a hydrophobic filler, such as hydrophobic fumed silicaparticles. These sensing compositions may further include one or moreemulsification enhancement agents, such as water soluble dextran andpolyvinyl alcohol. This patent discloses that a teflon sleeve is used onthe optical fiber during manufacture of the sensor to retain the sensingcomposition precursor on the optical surface of the optical fiber. Thesensor also includes an opaque overcoat, such as cellulose impregnatedwith carbon black, to optically isolate the sensing composition.

The use of disposable cassettes for blood analysis is of substantialcurrent interest, for example, to eliminate cross-patient contaminationand to keep more expensive components, e.g., optical and electroniccomponents, of the sensor system from being exposed directly to blood.One disposable cassette system is disclosed in commonly assigned U.S.patent application Ser. No. 229,617, filed Aug. 8, 1988.

Such a disposable cassette includes an indent, well or cavity into whicha sensing element is at least partially placed. The sensing element isexposed to blood and the sensing component, e.g., a fluorescent dye,gives off a signal which varies in response to variations in theconcentration of the gas of interest in the blood. A signal transmitter,e.g., an optical fiber, spaced away from the cassette well and bloodtransmits this signal to a processor where it is analyzed to provide thedesired blood gas concentration determination.

One problem which has presented itself with regard to the use of thesecassettes is that of inconsistent and/or inaccurate blood gasconcentration determinations. For example, such cassette blood gassensors have exhibited "drift"; that is, the signal representing acertain given gas concentration varies or drifts over time. Also, whenthe sensing element is manufactured or assembled in situ, e.g., in thewell of the cassette, there is no chance to screen the sensing elementbefore it is bonded to the cassette. Thus, non-specification sensingelements can be included in the cassette and determined as beingnon-conforming only after the assembled cassette, which often includes anumber of other sensing elements for different blood constituents, istested. At this point, the entire cassette must be discarded, resultingin substantial waste and cost.

It would be clearly advantageous to provide new sensing elements and/ornew methods for making sensing elements, particularly for use in suchcassettes.

SUMMARY OF THE INVENTION

A new sensing element, and method for making a sensing element, usefulin sensing the concentration of a gas, e.g., carbon dioxide, in amedium, e.g., blood, have been discovered. The present sensing elementsprovide substantial benefits. For example, these elements can beassembled, and then tested or screened, e.g., to assure conformity tospecifications, before they are included in the final sensor holder,e.g., cassette. This feature reduces the cost and time required toproduce high quality sensors which provide reliable and consistent gasconcentration determinations. Moreover, the present sensing elementspreferably have a reduced tendency to be subject to signal "drift". Thepresent method for producing sensing elements is straightforward andrelatively easy to practice. The relative amount of non-specificationsensing elements is reduced, and since the sensing element can be testedor screened before it is included with the other sensor components, thecost of producing non-specification sensing elements is even furtherreduced.

In one broad aspect, the present invention is directed to an assembledgas sensing element. This assembled element comprises a transparent, gasimpermeable, solid disc, preferably made of glass; an opaque, gaspermeable film, preferably including a polymer; and a quantity of a gassensing composition located between and in contact with the solid discand the film. This gas sensing composition comprises a gas permeable andlight permeable polymeric material, and a sensing component. Theassembled element is sized and adapted to be at least partially placedin a cavity or well, having one open end, in a sensor holder, e.g.,cassette, after assembly. Thus, this assembled element can be testedand/or otherwise screened before it is included in the sensor. This"pre-screening" feature reduces the amount of waste caused by having anoff specification sensing element. Thus, the unacceptable sensingelement alone can be discarded rather than having to scrap the entiresensor cassette.

In another broad aspect of the present invention, a method for making asensing element useful in sensing the concentration of a gas in a mediumis provided. This method comprises placing a quantity of a sensingcomposition precursor between and in contact with a transparent, gasimpermeable solid disc and an opaque, gas permeable film. The sensingcomposition precursor comprises a sensing component and a polymerprecursor. A sensing composition is formed from this quantity of sensingcomposition precursor.

After the sensing element has been produced, and preferably screened ortested to determine its efficacy, the sensing element is preferablyplaced at least partially in a cavity or well, having an open end andpreferably only one open end, located in a substantially transparentsensor holder. The sensing element is preferably oriented so that thefilm is exposed to the medium the gas concentration of which is beingmonitored and the face of the solid disc away from the sensingcomposition is substantially shielded from direct contact with thismedium.

Further aspects of the present invention provide a new sensingcomposition precursor and a new sensing composition. The sensingcomposition precursor comprises a dispersed aqueous liquid including asensing component dissolved therein, a polymer precursor, and ahydrophilic dispersing agent selected from hydroxyalkyl celluloses andmixtures thereof in an amount effective to facilitate maintaining thedispersed aqueous liquid. The sensing composition comprises a gaspermeable and light permeable polymeric material, a dispersed aqueousliquid including a sensing component dissolved therein, and ahydrophilic dispersing agent selected from hydroxyalkyl celluloses andmixtures thereof in an amount effective to facilitate maintaining theaqueous liquid dispersed in the sensing composition precursor prior toforming the sensing composition. The use of hydroxyalkyl celluloses, inparticular hydroxyethyl cellulose, rather than water soluble dextran orpolyvinyl alcohol as taught in the prior art, may contribute to thesubstantial reduction in the tendency of the sensors, in particular thecarbon dioxide sensors, of the present invention to be subject to signal"drift".

These and other aspects of the present invention are set forth in thefollowing detailed description and claims, particularly when consideredin conjunction with the accompanying drawings in which like parts bearlike reference numerals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view showing an embodiment of the presentsensing elements being assembled.

FIG. 2 is a cross sectional view showing an assembled sensing elementbeing bonded to a sensor cassette.

FIG. 3 is a schematic view showing a sensing element bonded to thesensor cassette in use providing signals useful to sense theconcentration of a gas in a medium.

DETAILED DESCRIPTION OF THE INVENTION AND DRAWINGS

The present invention is directed to an assembled gas sensing element,such as an assembled carbon dioxide sensing element or an assembledoxygen sensing element, useful in sensing the concentration of a gas ina medium, e.g., a fluid medium, in particular blood. This assembledsensing element comprises a transparent gas impermeable solid disc andan opaque gas permeable film. A quantity of a gas sensing composition islocated between and in contact with both the solid disc and the film.This gas sensing composition comprises a gas permeable and lightpermeable, and preferably substantially ion impermeable, polymericmaterial, and a sensing component. When the sensing composition isuseful in sensing carbon dioxide, it preferably includes a dispersedaqueous liquid including a sensing component dissolved therein. Theaqueous liquid in the present sensing composition is preferablydispersed in microcompartments, more preferably in microcompartmentshaving a maximum transverse dimension, e.g., diameter, of about 5microns or less. The sensing component preferably provides a signal,more preferably an optical signal, which varies in response tovariations in the concentration of the gas of interest in the mediumbeing monitored. The assembled gas sensing element is sized and adaptedto be at least partially placed into a cavity or well having one, andpreferably only one, open end and being located in a sensor holder,preferably a sensor cassette, after assembly.

The present sensing elements preferably have a reduced tendency to besubject to signal "drift". In particular, this reduced tendency isapparent in the present carbon dioxide sensing elements which are usedto periodically monitor carbon dioxide concentration in blood asfollows. Such carbon dioxide sensing elements are often stored in anenvironment having substantially no concentration of carbon dioxide. Inuse, the sensing elements are normally exposed to saline or othermaterials which include little or no carbon dioxide. Blood, having aconcentration (partial pressure) of carbon dioxide in the physiologicalrange, e.g., between about 20 and 60 mm Hg., is contacted with thesensing elements for up to about 5 minutes each time a concentrationdetermination is to be made. It is in this particularly difficult usethat the present sensing elements preferably show a reduced tendency tobe subject to signal "drift".

The solid disc is transparent and gas impermeable. Thus, the solid discallows the signal, preferably the optical signal, from the sensingcomponent to pass through, while reducing or even substantiallyeliminating diffusion of the gas of interest from the medium beingmonitored into the sensor holder. Such diffusion is disadvantageoussince it may detract from the accuracy of the gas concentrationdetermined. One particularly useful material of construction for thissolid disc is glass. The solid disc preferably includes opposing endsurfaces, spaced apart by the thickness of the disc, which aresubstantially mutually parallel. The disc is preferably shaped to atleast partially fit into the cavity of the sensor holder, as describedherein, e.g., with one end surface facing the wall opposite the open endof the sensor holder cavity or well. The disc can have the configurationof a right circular cylinder. In one embodiment, the disc has athickness in the range of about 0.005 inches to about 0.1 inches, andpreferably a diameter in the range of about 0.05 inches to about 0.5inches.

The film is opaque and gas permeable. This film acts to provide asubstantial degree of optical isolation for the sensing composition.This optical isolation allows a substantially focused or directed signalfrom the sensing component to be transmitted for analysis, as describedhereinafter, with reduced interference, e.g., from other sensingelements in the cassette. Ultimately, such optical isolation results inincreased accuracy and reliability of the gas concentrationdeterminations obtained using the present sensing elements. The filmshould be freely permeable to the gas of interest in the medium beingmonitored. This gas permeability allows the gas of interest from themedium to contact the sensing composition and interact with the sensingcomponent. The film is preferably made from a polymer combined with anopaque agent.

Any suitable polymer may be included in the presently useful film,provided that the polymer has no substantial detrimental effect on thefunctioning of the present system or on the medium being monitored. Thepolymer chosen preferably provides a relatively thin film withsufficient structural integrity and durability to be useful in thepresent methods of producing sensing elements, as well as in use sensinggas concentrations as part of the final sensing element. In oneparticularly useful embodiment, the film includes a fluorine-containingpolymer, preferably a polymer selected from polyfluorohydrocarbons,polyfluorocarbons and mixtures thereof, and especially,polytetrafluoroethylene. Preferably, the film is substantially thinnerthan the solid disc, e.g., to facilitate gas permeability. Morepreferably, the thickness of the film is in the range of about 1% toabout 20% of the thickness of the solid disc. Like the solid disc, thefilm preferably includes opposing end surfaces which are substantiallymutually parallel. Such end surfaces are preferably shaped to becompatible with the shape of the solid disc and with the shape of thecavity of the sensor holder. Such end surfaces may be circular. In oneembodiment, the film has a thickness in the range of about 0.0001inches, or less, to about 0.001 inches, or more, preferably about 0.0002inches to about 0.0006 inches.

Any suitable opaque agent may be used provided that such agent or agentsfunction to provide the desired degree of optical isolation and have nosubstantial detrimental effect on the functioning of the present systemor on the medium being monitored. Among the opaque agents useful in thepresent invention are carbon black, other carbon based opaque agents,ferric oxide, metallic phthalocyanines and the like. Such opaque agentsare preferably substantially uniformly dispersed in the film in anamount effective to provide the desired degree of opacity, e.g., toprovide the desired optical isolation. A particularly useful opaqueagent is carbon black.

The polymeric material of the gas sensing composition is permeable tothe gas of interest in the medium being monitored and is permeable tothe wave lengths of light utilized in the measurement of theconcentration of this gas. Further, if the sensing composition is to beused in sensing carbon dioxide, this polymeric material is preferablyimpermeable to ions and to liquid water. For example, if the sensingcomponent is dissolved in a dispersed aqueous liquid and, as ispreferred, the aqueous liquid contains a buffer, the concentration ofthe buffer ions is preferably maintained substantially constant so thatthe sensing composition provides consistent signals in response to theconcentration of the gas of interest in the medium.

Any suitable polymeric material may be employed in the present gassensing compositions provided that the polymeric material has nosubstantial detrimental effect on the functioning of the present systemor on the medium being monitored. Because of substantial gas and lightpermeability and aqueous impermeability properties, silicone-basedpolymeric materials are preferred. More preferably, cross-linkedsilicone-based polymeric materials are employed. When the gas ofinterest is carbon dioxide or oxygen, the polymeric material ispreferably cross-linked polydimethyl siloxane. The precursor of thepolymeric material, hereinafter referred to as the polymer precursor,may be selected from one or more monomers, pre-polymers, and mixturesthereof. A particularly useful polymer precursor, e.g., when carbondioxide or oxygen is the gas of interest, is vinyl terminated dimethylsiloxane, such as that sold by Petrarch Systems under the trademark PS443. If the polymeric material is to be cross-linked, a cross-linkingagent is included with the polymer precursor. Such cross-linking agentsare preferably compounds which include at least two functional groupscapable of reacting with the polymer precursor and/or a partiallypolymerized intermediate to form cross links, e.g., between polymerchains, in the polymeric material. A particularly useful cross-linkingagent is methylhydro-dimethylsiloxane copolymer, such as that sold byPetrarch Systems under the trademark PS 123, especially when the polymerprecursor includes vinyl terminated dimethylsiloxane. One or morecatalysts may be used to promote the formation of the polymericmaterial. One such catalyst is platinum. An example of aplatinum-containing catalyst material is sold by Petrarch Systems underthe trademark PC072. The amount of catalyst used should be sufficient topromote the desired degree of polymerization. Of course, the catalystshould have no substantial detrimental effect on the functioning of thepresent system or on the medium being monitored.

Alternately, the silicone-based polymeric material can be formed throughcondensation polymerization reactions with silanol terminated siliconesbeing cross-linked with alkoxyl silanes using catalysts, such as tinderivatives.

Fillers can be, and preferably are, included in the present gas sensingcompositions which include dispersed aqueous liquid. Such fillers act toenhance the stability of the dispersed aqueous liquid in the sensingcomposition and the strength of the sensing composition. Any suitablefiller may be used provided it has no substantial detrimental effect onthe functioning of the present system or on the medium being monitored.In one embodiment, the filler has a hydrophobic nature. Such fillers arepreferably present in an amount in the range of about 1% to about 20% byweight, based on the amount of polymeric material present in the gassensing composition. A particularly useful filler is hydrophobic fumedsilica, e.g., in the form of fine particles.

The sensing component or components chosen for use are those effectiveto provide a signal which varies in response to variations in theconcentrations of the gas of interest in the medium being monitored. Thesensing component is preferably an optical indicator, such asfluorescence indicators and absorbance indicators, in particularfluorescence indicators. In sensing carbon dioxide concentrations,examples of absorbance indicators that can be used include chlorophenylred, bromo cresol purple, nitrophenol, bromo thymol blue, penachlorome,pheno red and the like. Useful fluorescence indicators for carbondioxide include hydroxypyrene 3,6,8-trisulfonic acid, herein referred toas HPTS or hydroxypyrene trisulfonic acid, derivatives, e.g., salts, ofHPTS, beta-methylumbelliferone, fluorescein and the like. The morepreferred sensing component, particularly for sensing the concentrationof carbon dioxide in blood, is selected from HPTS, derivatives of HPTSand mixtures thereof. The alkali and alkaline earth metal salts of HPTSare useful HPTS derivatives. Very pure sensing components, in particularlaser grade HPTS and derivatives of laser grade HPTS, are very effectivefor producing carbon dioxide sensing elements which advantageously havea reduced tendency to be subject to signal drift over time. In sensingoxygen concentrations, examples of fluorescence indicators include oneor more polynuclear aromatic compounds, derivatives of polynucleararomatic compounds and the like. Examples of such polynuclear aromaticcompounds include decacyclene, benzo-ghi-perylene and coronene. Theoxygen indicators may include a mixture of tertiary butyl derivatives ofsuch polynuclear aromatic compounds. Such indicators are more fullydescribed in Yafuso, et al U.S. Pat. No. 4,849,172 which is incorporatedby reference in its entirety herein.

The amount of sensing component used is such as to provide asufficiently strong signal so that the concentration of the gas ofinterest can be reliably and accurately determined.

For sensing carbon dioxide, the aqueous liquid of the gas sensingcomposition is preferably buffered, more preferably using a carbonateion and/or bicarbonate ion based buffer system. Such a buffer ispreferably chosen so as to have a buffer range compatible with theresponse range of the sensing component. Such a range might, forinstance, mimic the physiological pH range of blood. Any suitable buffermaterial may be employed to buffer the aqueous liquid, provided suchmaterial has no substantial detrimental effect on the functioning of thepresent system or on the medium being monitored. Examples of suchmaterials include alkali metal carbonates, alkali metal bicarbonates andthe like. The amount of buffer used is such as to provide the desireddegree of buffering to the aqueous medium. For measuring blood carbondioxide with hydroxypyrene trisulfonic acid, a pH range of about 6 toabout 8 is desirable. In one embodiment, the sensing component iseffective to provide a signal which varies as the pH of the aqueousliquid varies.

The present gas sensing composition precursors and gas sensingcompositions which include an aqueous liquid preferably also include aneffective amount of at least one hydrophilic dispersing agent. Suchagents act to facilitate maintaining the aqueous liquid dispersed in thesensing composition precursor prior to forming the polymeric material.Thus, increases in shelf life of the sensing composition precursor areobtained. Also, these agents may act to retard dehydration of theaqueous liquid. Overall, such hydrophilic dispersing agents reduce theneed to adhere to a tight manufacturing schedule in producing theassembled sensing elements of the present invention, and reduce thegeneration of manufacturing "scrap materials" which is economicallywasteful.

Any suitable hydrophilic dispersing agent may be used provided that suchagent has no substantial detrimental effect on the functioning of thepresent system or on the medium being monitored. Examples of hydrophilicdispersing agents include water-soluble dextran, polyvinyl alcohol andthe like materials. The amount of hydrophilic dispersing agent utilizedis not critical and may vary depending, for example, on the specificdispersing agent, polymer precursor and filler being employed. In oneembodiment, the amount of dispersing agent present is in the range ofabout 1% to about 20% by weight of the total aqueous liquid employed.

One of the important preferred features of the present invention is theuse of hydrophilic cellulosic dispersing agents rather than the watersoluble dextran or polyvinyl alcohol suggested in the prior art, e.g.,Yafuso et al U.S. Pat. No. 4,824,789. Such cellulosic agents, which arepreferably water soluble, provide gas sensing elements which provide aconsistent signal in response to a given concentration of gas in theliquid medium being monitored. The hydrophilic cellulosic materials usedas dispersing agents are preferably hydroxyalkyl celluloses and mixturesthereof. The alkyl group may contain, for example, one to about five ormore carbon atoms. A particularly useful dispersing agent ishydroxyethyl cellulose.

The precursor for the carbon dioxide sensing composition may be preparedas follows. The precursor for the oxygen sensing composition may beprepared in a somewhat analogous manner except that no aqueous liquid isused and an organic solvent is used to combine the sensing componentinto the polymer precursor. The carbon dioxide sensing component isdissolved in a quantity of liquid water to form an aqueous liquid. Ifdesired, a buffer and/or hydrophilic dispersing agent can also bedissolved in this aqueous liquid. The desired amount of polymerprecursor, and, if desired, a filler component and polymerizationcatalyst, is (are) mixed with the aqueous liquid. At this point, nocross-linking agent or agents are included.

The resulting mixture is subjected to strong agitation, e.g., in amechanical homogenizer, to disperse the aqueous liquid and form a firstsensing composition precursor. After this agitation, the first sensingcomposition precursor containing dispersed aqueous liquid can be storedfor a period of time, e.g. on the order of about 1 hour to about 24hours or more, ready for use in making the assembled gas sensingelement.

When it is desired to form the gas sensing composition of the invention,the cross-linking agent or agents, if any, and the polymerizationcatalyst if not already present, are added to the first sensingcomposition precursor. These are gently stirred into the first sensingcomposition precursor to form the second or final gas sensingcomposition precursor.

After the final sensing composition precursor has been prepared, aquantity of it is placed on one side of a sheet of the opaque film. Atleast one solid disc is then placed on this quantity of final sensingcomposition precursor so that the final sensing composition precursor islocated between and in contact with both the solid disc and the opaquefilm. Preferably, a plurality of such solid discs are so placed, morepreferably placed spaced apart, on this sensing composition precursor.If a plurality of solid discs are used, the sensing compositionprecursor may be located in individual or discrete portions, e.g. drops,on the opaque film. In this embodiment, a plurality of gas sensingelements can be effectively produced from a single sheet of opaque film.The sensing composition precursor is then allowed to react, e.g.,polymerize and/or cross-link, to form the sensing composition. Theassembled sensing element can be obtained by removing, e.g, cutting, thesolid disc, together with a portion of the sensing composition and aportion of the opaque film sheet from the remainder of the opaque filmsheet. Additional processing, e.g., trimming, may be useful tosubstantially conform the shape of the opaque film and sensingcomposition components of the sensing element to the shape of the areaof the solid disc in contact with the sensing composition component. Theassembled gas sensing element can be tested and/or screened, usingconventional techniques, to determine its efficacy in measuring theconcentration of the gas of interest. Such tests, for example, may seekto determine the intensity of the signal provided by the assembled gassensing element and/or the response time of the assembled gas sensingelement to changes in the concentration of the gas of interest.

If the assembled gas sensing element is satisfactory, it is then placedat least partially in an open-ended cavity in a sensor holder.Preferably, the assembled sensing element is secured, e.g., by adhesive,to the sensor holder. The sensing element is situated relative to thesensor holder so that the solid disc is preferably located relativelynear to and facing the bottom of the open-ended cavity and the opaquefilm is exposed to the medium being monitored.

The sensor holder is preferably placed in proximity to an optical fibersuch that signals from the sensing composition can be transmitted by theoptical fiber to a processor so that the concentration of the gas ofinterest in the medium can be determined based upon such signals.

The following non-limiting examples illustrate certain aspects of thepresent invention.

EXAMPLE 1

An aqueous solution was formed by dissolving 0.2 g of laser gradehydroxypyrene trisulfonate (trisodium salt), 0.25 g of sodium chloride,0.042 g of sodium carbonate and 6.1 g of hydroxyethyl cellulose in 60 ccof water.

A silicone formulation was made by mixing 0.3 g of fine particles ofhydrophobic fumed silica, sold by Tulco, Inc. under the trademarkTulanox 500, into 10 g of vinyl terminated dimethyl siloxane, sold byPetrarch Systems under the trademark PS 443. This mixture also includeda trace amount (1 drop per 50 g of the vinyl terminated dimethylsiloxane) of a platinum-containing component (catalyst), sold byPetrarch Systems under the trademark PC 072.

2.0 g of the aqueous solution was combined with the silicone formulationand the mixture was homogenized in a Virtis 25 homogenizer. An amount ofmethylhydro-dimethylsiloxane copolymer, sold by Petrarch Systems underthe trademark PS 123, equal to about 10% by weight of the homogenizedmixture was mixed with the homogenized mixture to form the sensingcomposition precursor.

A 10 cm by 10 cm sheet of an opaque film, about 0.0004 inches thick, wasstretched on a frame or fixture. The opaque film was made frompolytetrafluoroethylene mixed with carbon black and sold by ChemicalFabrics Corporation under the trademark DF-1100 Black teflon. Discretedrops of the sensing composition precursor was applied to the bondableside of the stretched sheet.

A right circular cylindrical transparent glass disc, about 0.012 inchesthick and 0.120 inches in diameter, was placed on each drop of thesensing composition precursor, with a flat end directly contacting theprecursor. These discs had previously been treated with allyl trimethoxysilane, a coupling agent, and a solution of platinum-containing catalystin hexane. With the discs in place, the resulting composite was allowedto stand overnight at room temperature and then the vinyl terminateddimethyl siloxane and methylhydrodimethylsiloxane copolymer mixture wasfurther cured for about 90 minutes at 65° C. to form a sensingcomposition comprising cross-linked silicone-based polymer withdispersed microcompartments of the aqueous solution.

Individual carbon dioxide sensing elements were cut from the glassdiscs/sensing composition/sheet composite. Each sensing element includedone glass disc, a generally circularly shaped portion of an opaque filmand a thin layer of the sensing composition therebetween. The sensingelement was trimmed to more closely conform the opaque film and thelayer of sensing composition to the circular shape of the glass disc.The end of the glass disc not in contact with the sensing compositionwere washed with methanol.

Using conventional testing procedures, each of the assembled carbondioxide sensing elements is tested to determine its efficacy foraccurately and reliably sensing the concentration of carbon dioxide inhuman blood. These assembled carbon dioxide sensing elements are foundto be satisfactory based on this testing procedure.

Using a transparent silicone-based adhesive, one of these carbon dioxidesensing elements is bonded into a right circular cylindrical open endedwell, having a diameter of 0.125 inches and a depth of 0.010±0.001inches, formed in a polycarbonate cassette. The glass disc is facing thebottom of the well. Prior to bonding the sensing elements to thecassette, the walls of the well are contacted with a priming agent, soldby Dow Chemical Company under the trademark Dow 1205, to promoteadhesion between the sensing element and the polycarbonate cassette.

The thus produced carbon dioxide sensor is effective in determining theconcentration of carbon dioxide in blood brought into contact with thefilm.

FIGS. 1, 2 and 3 illustrate the manufacture of a sensing element asdescribed in the above Example, and its use in determining gasconcentrations. In FIG. 1, the relative thicknesses of the glass discsand opaque film are somewhat accurately illustrated, with the disc beingmuch thicker than the film. The sensing composition between the discsand film is of intermediate thickness. In FIGS. 2 and 3, for the sake ofillustration clarity, the thicknesses of all of the glass disc, opaquefilm and sensing composition are shown as being equal.

Referring now to FIG. 1, the disassembled gas sensing element, showngenerally at 10, includes a stretched sheet 12 of opaque film, glassdiscs 14 and 15 and drops 16 and 17 of the sensing compositionprecursor. As shown in FIG. 1, the drops 16 and 17 of sensingcomposition precursor are placed on the stretched sheet 12. The glassdiscs 14 and 15 are then placed on top of the drops 16 and 17,respectively of sensing composition precursor and this precursor isallowed to cure to form the sensing composition. Referring to FIGS. 2and 3, after being cut from the stretched sheet 12 and trimmed toconform to the circular shape of glass disc 14, the assembled gassensing element, shown generally at 20, includes an opaque film 21, thesensing composition 22 and the glass disc 14. The sensing composition 22is secured to both the opaque film 21 and the glass disc 14.

The sensing element 20 is tested and found to have acceptablecharacteristics for use in a sensor for measuring the concentration ofcarbon dioxide in blood.

As shown in FIGS. 2 and 3, this assembled gas sensing element 20 isplaced into well 24 of sensor holder 26 and based adhesive. Well 24 isopen at one end, includes a right circular cylindrical side wall 25 anda circular bottom end wall 28. The size of well 24 is such that theassembled gas sensing element 20 and silicone-based adhesive layer 27completely fill well 24. Assembled gas sensing element 20 is placed inwell 24 so that the glass disc 14 faces the bottom end wall 28 of well24. The opaque film 12 includes an exposed surface 29 which is raisedrelative to the inner surface 30 of sensor holder 26. The opaque film 12substantially shields sensing composition 22 from direct contact withthe medium, e.g., blood, to be monitored.

Referring now to FIG. 3 in use sensor holder 26, made of a transparentpolycarbonate material is placed in abutting relation to optical fiber32. Optical fiber 32 provides excitation light of appropriate wavelengthfrom light transmitting apparatus 34 to excite the sensing component inthe sensing composition 22 to fluoresce and provide a signalcharacteristic of the concentration of carbon dioxide located in themedium in contact with the opaque film 12. This optical fiber 32 alsotransmits the signal which is emitted from the sensing component andpasses such signal to a light receiving apparatus 36, which processes oranalyzes this emitted signal, e.g., as described in Lubbers et alReissue U.S. Pat. No. 31,879 and Heitzmann U.S. Pat. No. 4,557,900 36,to determine the concentration of carbon dioxide in this medium. Theabove-noted Heitzmann patent is incorporated by reference in itsentirety herein.

Over a period of time the assembled sensing element 20 providesconsistent, e.g., substantially "drift" free, signals which are reliablycorrelated to the true and accurate concentration of carbon dioxide inthe blood in contact with the opaque film 12.

EXAMPLE 2

Example 1 is repeated except that the sensing composition precursorincludes a fluorescence oxygen indicator, vinyl terminateddimethylsiloxane and a silicone-based cross-linking agent.

The assembled sensing element produced in Example 2 is used as shown inFIG. 3 and as described above with regard to the carbon dioxide sensingelement except that the Example 2 sensing element is used to sense theoxygen concentration in the blood. Over a period of time this Example 2assembled sensing element provides acceptable signals which are reliablycorrelated to the true and accurate concentration of oxygen in the bloodin contact with the opaque film.

While this invention has been described with respect to various specificexamples and embodiments, it is to be understood that the invention isnot limited thereto and it can be variously practiced within the scopeof the following claims.

What is claimed is:
 1. A sensing composition precursor comprising adispersed aqueous liquid including a sensing component dissolvedtherein, a polymer precursor, and a hydrophilic dispersing agentselected from the group consisting of hydroxyalkyl celluloses andmixtures thereof in an amount effective to facilitate maintaining saiddispersed aqueous liquid.
 2. The sensing composition precursor of claim1 wherein said hydrophilic dispersing agent is hydroxyethyl cellulose.3. The sensing composition precursor of claim 1 wherein said polymerprecursor is a vinyl terminated dimethyl siloxane and said sensingcomposition precursor further includes a cross-linking agent.
 4. Thesensing composition precursor of claim 1 wherein said sensing componentis selected from the group consisting of hydroxypyrene trisulfonic acid,derivatives of hydroxypyrene trisulfonic acid and mixtures thereof.
 5. Asensing composition comprising:a gas permeable and light permeablepolymeric material, a dispersed aqueous liquid including a sensingcomponent dissolved therein, and a hydrophilic dispersing agent selectedfrom the group consisting of hydroxyalkyl celluloses and mixturesthereof in an amount effective to facilitate maintaining aqueous liquiddispersed in a precursor of said sensing composition prior to formingsaid sensing composition, said precursor comprising a dispersed aqueousliquid including said sensing component dissolved therein, a polymerprecursor and said hydrophilic dispersing agent.
 6. The sensingcomposition of claim 5 wherein said hydrophilic dispersing agent ishydroxyethyl cellulose.
 7. The sensing composition of claim 5 whereinsaid polymeric material is substantially ion impermeable, said aqueousliquid is buffered and said sensing component is effective to provide asignal which varies as the pH of said aqueous liquid varies.
 8. Thesensing composition of claim 5 wherein said aqueous liquid has a pH inthe range of about 6 to about 8 and wherein said sensing composition iseffective in sensing the concentration of carbon dioxide in blood. 9.The sensing composition precursor of claim 1 wherein said hydrophilicdispersing agent is soluble in said dispersed aqueous liquid.
 10. Thesensing composition precursor of claim 5 wherein said hydrophilicdispersing agent is soluble in said dispersed aqueous liquid.